Co‐oligopeptides of aromatic amino acids and glycine with a variable distance between the aromatic residues. VI. Circular dichroism studies of co‐oligopeptides of L‐phenylalanine, L‐tryptophan, and glycine

The circular dichroic properties of H‐Gly‐Phe‐(Gly) n ‐Trp‐Gly‐OH (II, n = 0,1,2) and of related simpler peptides, such as H‐Phe‐Gly‐OH, H‐Gly‐Phe‐OH, H‐Gly‐Phe‐Gly‐OH, H‐Phe‐Trp‐OH, H‐Phe‐Trp‐Gly‐OH, and H‐Gly‐Phe‐Trp‐OH in water and trifluoroethanol solutions are investigated. Peptides containing only one phenylalanyl residue show markedly different near‐uv dichroic signals depending on whether this residue is in the N‐terminal position or not. The possible origin of this feature is discussed. The study of the oligopeptides II ( n = 0,1,2) shows that no strong intramolecular interaction between the two aromatic rings is present. However, the dichroic properties of II ( n = 0) are clearly anomalous, and the analysis of H‐Gly‐Phe‐Trp‐OH, H‐Phe‐Trp‐Gly‐OH, and of H‐Phe‐Trp‐OH at different pH's, confirms that the presence of two adjacent aromatic residues may bring about chiroptical properties which indicate a restriction in the conformational equilibrium of the molecule. The limits, and the possible generalization of this finding, are discussed.